The invention relates to a method of operation for a spark-ignition, direct-injection internal combustion engine with at least one ignition device entering the cylinder chamber and at least one injection device entering the cylinder chamber, in which the main fuel injection into the cylinder chamber takes place during the intake stroke.
In direct-injection combustion engines with spark ignition fuel is usually injected into the combustion chamber in such a way that a fuel-rich ignitable mixture occurs in the vicinity of the ignition device while a lean fuel air mix prevails elsewhere on average. In this way charge stratification of fuel-rich mixture in an otherwise lean mixture is achieved, enabling the richer mixture to be ignited, while still permitting operation with an air ratio xcex greater than 1. Due to thorough combustion and favorable thermodynamic conditions owing to unthrottled intake flow high efficiency and low fuel consumption are attained. Practical experience has shown, however, that real fuel consumption lags behind theoretical expectation. A reason for this discrepancy is that in conventional wall-guided systems relatively large heat losses will occur at the walls due to the extreme enlargement of the fuel-wetted piston top surface. Furthermore, the high kinetic energy of the charge required due to charge interchange losses cannot be neglected. Besides, conventional serially produced direct-injection, spark-ignition engines will produce high NOx-emissions due to the large proportion of approximately stoichiometric fuel-air mixtures in the spraycloud, such emissions demanding costly after-treatment.
From DE 44 41 092 A1 a valve for introducing fuel or fuel-air mixture into the combustion chamber of a combustion engine is known, by means of which an umbrella-shaped or cone-shaped injection jet is generated. To insure ignition of the fuel-air mixture contained in the combustion chamber, an ignition jet is branched off from the umbrella-shaped injection jet, which, on entering the immediate vicinity of the spark plug, causes a fuel-rich fuel-air mixture to be present in the area of the ignition spark. The intention is to achieve a lesser degree of wall wetting and thus a lower HC-content of the exhaust gases and lower fuel consumption.
It is the object of the present invention to propose an operating method by which fuel consumption and emissions may be optimized in direct-injection, spark-ignition, internal combustion engines.
According to the invention, this object is achieved by providing that, during idling operation and/or under partial load conditions, a homogeneous fuel-air mixture with an air ratio xcex greater than 2 is established in the cylinder chamber by the air-distributed main injection process, and during the compression stroke an air-atomized fuel ignition jet is injected into the cylinder chamber such that an ignitable, preferably stoichiometric, fuel air mixture is produced in the vicinity of the ignition device and the homogeneous fuel air mixture of the main injection process is ignited by the ignition jet. Thus, during idling and/or under partial load conditions, a homogeneous fuel air mixture introduced at an earlier point in time and having an air ratio xcex greater than 2 is ignited by an optimally atomized ignition jet.
The combustion process is similar to that of a conventional spark-ignition engine, though characterised by relatively fast propagation of the flame front, which starts out centrally at the fuel-rich ignition core and propagates outwardly with a progressive rise in temperature.
Particularly low emissions of nitrogen oxides and low fume values may be achieved if the injected volume of the ignition jet is a fraction of the main injection volume, preferably at most 10%, and preferably about 5-10% of the main injection volume. Since NOx or fume emissions are generated almost exclusively in the area of the roughly stoichiometric ignition core, which may be kept very small due to the small amount of fuel in the ignition jet, a substantial improvement of exhaust gas quality may be achieved as compared with conventional direct injected spark ignition engines.
It is particularly advantageous if the combustion engine is unthrottled under partial load and exhaust gas is recycled at least under partial load, such that the air ratio xcex of the fuel-air mixture of the main injection is set by the amount of recycled exhaust gas. Setting of the air ratio xcex of the fuel-air mixture introduced by the main injection process may be effected in a simple way by controlling the volume of exhaust gas recycled. Temperature and rate of the recycled exhaust gas may also be used to control combustion speed.
To achieve optimum atomisation of the ignition jet, fuel should be introduced into the cylinder chamber at sufficiently high injection pressure. This may be effected by using an accumulator injection system, for instance, or a pump-and nozzle injection system.
In order to achieve uniform homogenisation of the fuel introduced during the main injection process, it is of particular advantage if a tumble motion of the charge in the cylinder chamber is generated at least during main injection. A tumble flow is a cylindrical flow in the cylinder chamber when the axis of rotation is approximately normal to the cylinder axis. The tumble flow will enhance mixture formation in the main injection phase.